Polarization-insensitive phase modulator

ABSTRACT

An optical device ( 20 ) includes an electro-optical layer, including a liquid crystal material ( 24 ) with a heliconical structure having a pitch that is less than 250 nm and is modifiable by an electric field. An array of excitation electrodes ( 28 ) extends over the electro-optical layer. Control circuitry ( 23 ) is coupled to apply control voltage waveforms to the excitation electrodes and is configured to modify the control voltage waveforms so as to locally modify a molecule director angle of the heliconical structure and thus to generate a specified phase modulation profile in the electro-optical layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication 62/308,903, filed Mar. 16, 2016, which is incorporatedherein by reference. This application is a Continuation in Part of U.S.patent application Ser. No. 14/428,426, filed in the national phase ofPCT Patent Application PCT/1132013/058989, filed Sep. 30, 2013, whichclaims the benefit of U.S. Provisional Patent Application 61/707,962,filed Sep. 30, 2012, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to electro-optical devices, andparticularly to optical phase modulators.

BACKGROUND

A dynamic phase modulator is an optical device that allows phasemodulation of transmitted light, wherein the phase modulation iselectronically controllable. Among its various applications are opticalmodulators for communication and wave front shaping for optical usessuch as microscopy, astrophysics and optometry. One sought-afterapplication is an electrically-controlled dynamic lens.

U.S. Pat. No. 9,335,562 describes methods and apparatus for providing avariable optic insert into an ophthalmic lens. A liquid crystal layermay be used to provide a variable optic function, and in someembodiments the liquid crystal layer may comprise polymer networkedregions of interstitially located liquid crystal material.

SUMMARY

Embodiments of the present invention that are described hereinbelowprovide phase modulators that are independent of the polarization ofincident light.

There is therefore provided, in accordance with an embodiment of theinvention, an optical device, which includes an electro-optical layer,including a liquid crystal material with a heliconical structure havinga pitch that is less than 250 nm and is modifiable by an electric field.An array of excitation electrodes extends over the electro-opticallayer. Control circuitry is coupled to apply control voltage waveformsto the excitation electrodes and is configured to modify the controlvoltage waveforms so as to locally modify a molecule director angle ofthe heliconical structure and thus to generate a specified phasemodulation profile in the electro-optical layer.

In a disclosed embodiment, the liquid crystal material includes acombination of one or more liquid crystals and a chiral additive.Alternatively, the liquid crystal material comprises a chiral liquidcrystal.

Typically, the phase modulation profile is independent of a polarizationof the incident light.

In some embodiments, the control circuitry is configured to apply thecontrol voltage waveforms to the excitation electrodes so that thedevice functions as a lens, having focal properties determined by thephase modulation profile, for example an ophthalmic lens with anelectrically controllable focal length.

There is also provided, in accordance with an embodiment of theinvention, an optical device, including a first polarization-dependentlens, which include a first electro-optical layer, which is configuredto refract a first polarization component of light propagating along anoptical path, with an effective first local index of refraction at anygiven location that is determined by first control voltage waveformsapplied across the first electro-optical layer, and a first array ofexcitation electrodes extending across the first electro-optical layer.A second polarization-dependent lens is arranged in series with thefirst polarization-dependent lens along the optical path and includes asecond electro-optical layer, which is configured to refract a secondpolarization component of the light, orthogonal to the firstpolarization component, with an effective second local index ofrefraction at any given location that is determined by second controlvoltage waveforms applied across the second electro-optical layer, and asecond array of excitation electrodes extending across the secondelectro-optical layer. Control circuitry is coupled to apply the firstand second control voltage waveforms respectively to the first andsecond arrays of the excitation electrodes and is configured to modifythe first and second control voltage waveforms so as to generate aspecified phase modulation profile in the first and secondelectro-optical layers.

In some embodiments, the specified phase modulation profile includes afirst phase modulation profile that is generated in the firstelectro-optical layer by the first control voltage waveforms and definesa first focal length of the first lens and a second phase modulationprofile that is generated in the second electro-optical layer by thesecond control voltage waveforms and defines a second focal length ofthe second lens. Typically, the first and second polarization-dependentlenses are positioned at respective locations that are separated by apredefined distance along the optical path, and the first and secondcontrol voltage waveforms are selected to determine the first and secondfocal lengths so that a difference between the first and second focallengths compensates for the predefined distance in forming respectivefirst and second images at a focal plane. In a disclosed embodiment, thefirst and second polarization-dependent lenses are configured to serveas an ophthalmic lens with an electrically-controllable focal length,and the first and second focal lengths are chosen so that both the firstand second polarization components are imaged on a retina of a user ofthe ophthalmic lens with equal magnifications.

There is additionally provided, in accordance with an embodiment of theinvention, a method for producing an optical device. The method includesproviding an electro-optical layer, including a liquid crystal materialwith a heliconical structure having a pitch that is less than 250 nm andis modifiable by an electric field. An array of excitation electrodes ispositioned to extend over the electro-optical layer. Control circuitryis coupled to apply control voltage waveforms to the excitationelectrodes and to modify the control voltage waveforms so as to locallymodify a molecule director angle of the heliconical structure and thusto generate a specified phase modulation profile in the electro-opticallayer.

There is further provided, in accordance with an embodiment of theinvention, a method for producing an optical device. The method includesproviding a first polarization-dependent lens, which includes a firstelectro-optical layer, which is configured to refract a firstpolarization component of light propagating along an optical path, withan effective first local index of refraction at any given location thatis determined by first control voltage waveforms applied across thefirst electro-optical layer, and a first array of excitation electrodesextending across the first electro-optical layer. A secondpolarization-dependent lens is arranged in series with the firstpolarization-dependent lens along the optical path. The secondpolarization-dependent lens includes a second electro-optical layer,which is configured to refract a second polarization component of thelight, orthogonal to the first polarization component, with an effectivesecond local index of refraction at any given location that isdetermined by second control voltage waveforms applied across the secondelectro-optical layer, and a second array of excitation electrodesextending across the second electro-optical layer. Control circuitry iscoupled to apply the first and second control voltage waveformsrespectively to the first and second arrays of the excitation electrodesand to modify the first and second control voltage waveforms so as togenerate a specified phase modulation profile in the first and secondelectro-optical layers.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of an optical device, inaccordance with an embodiment of the present invention;

FIG. 2 is a schematic perspective view of the structure of a heliconicalliquid crystal material used in a polarization-independent optical phasemodulator, in accordance with an embodiment of the present invention;

FIG. 3 is a schematic perspective view of a polarization-independentoptical device, in accordance with an embodiment of the presentinvention; and

FIGS. 4A-B are schematic side views of a polarization-independentophthalmic lens, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

When dynamic lenses, for example the dynamic lens described in theabove-mentioned PCT Patent Application PCT/IB2013/058989, are based onbirefringent materials, such as nematic liquid crystals, they typicallyoperate only on polarized light. In order to modulate the phases of bothpolarizations, two such devices can be stacked with their directions ofpolarization crossed. In this case, however, the necessary spacingbetween the two devices affects the relative magnifications of theimaging system in the two polarizations.

As another possible solution to the problem of polarization-dependence,a dynamic lens could be based on cholesteric liquid crystals (CLC),which have an axially symmetrical helical structure when no voltage isapplied across the layer of liquid crystal. In this state the opticalpath length across the CLC, and therefore the optical phase change, isindependent of the polarization of the incident light. However, when avoltage is applied across a traditional CLC material, the liquid crystalmaterial switches to an intermediate state, such as a focal conic stateor a fingerprint state. These states are disordered and thereforescatter the incident light. Further increasing the voltage results in ahomeotropic state, in which all the molecules are mutually parallel,with the common direction of the molecules perpendicular to the walls ofthe liquid crystal cell. As a result, continuous electronic control ofthe focal properties of a dynamic lens based on a traditional CLCmaterial is generally not feasible.

The embodiments of the present invention that are described hereinaddress these problems by providing electronically controllable opticsthat are polarization-independent.

In some embodiments of the present invention, polarization-independenceis achieved using a novel liquid crystal material with a heliconicalstructure having a pitch that is less than 250 nm. The pitch anddirector angle of the liquid crystal molecules are modifiable by anelectric field. An array of excitation electrodes extends over the layerof liquid crystal. Control circuitry is coupled to apply control voltagewaveforms to the excitation electrodes and is configured to modify thesewaveforms so as to modify the director angle and and/or pitch of theliquid crystal material and thus generate a specified phase modulationprofile in the liquid crystal layer.

In other embodiments of the present invention, thepolarization-independence is achieved by stacking twopolarization-dependent lenses in series along the optical path, withorthogonal polarization axes. Each lens comprises an electro-opticallayer with an array of excitation electrodes extending over the layer.Control circuitry is coupled to apply control voltage waveforms to thearrays of excitation electrodes and is configured to modify the controlvoltage waveforms so as to generate a specified phase modulation profilein each of the electro-optical layers. In some of these embodiments, thecontrol voltage waveforms are chosen so that the two lenses havedifferent focal powers, so that both polarizations are imaged with thesame magnification to avoid a doubling of the image.

Polarization-Independence Using Heliconical Liquid Crystals

FIG. 1 is a schematic perspective view of an optical device 20, inaccordance with an embodiment of the present invention. Optical device20 comprises an optical phase modulator 22 and control circuitry 23.Optical phase modulator 22 comprises a layer of a heliconical liquidcrystal material 24 sandwiched between an upper substrate 26 and a lowersubstrate 27, wherein the substrates comprise a transparent material,for example, glass. Substrates 26 and 27 can be coated on their insideswith a polyimide alignment layer, for example PI-1211, produced byNissan Chemical Industries Ltd., Japan (not shown). Liquid crystalmaterial 24 is typically contained by suitable encapsulation, as isknown in the art.

Light impinges on optical phase modulator 22 as an incident light 30,and exits the phase modulator as a transmitted light 32. The opticalphase of transmitted light 32 is locally modified, with respect to theoptical phase of incident light 30, by the local optical path throughliquid crystal material 24. The local optical path is modified inresponse to applied control voltage waveforms, as will be detailedbelow.

Excitation electrodes 28 and 29 are disposed respectively oversubstrates 26 and 27. Excitation electrodes 28 and 29 comprise atransparent, conductive material, such as indium tin oxide (ITO), as isknown in the art. Alternatively, non-transparent excitation electrodesmay be used, as long as they are thin enough so that they do not causedisturbing optical effects.

Excitation electrodes 28 in this embodiment are arranged as an array ofparallel stripes. (“Parallel” in this context may include, as well,excitation electrodes that deviate in angle by several degrees.) Forexample, the electrode pattern shown in FIG. 1 may be formed bylithography on substrate 26. Although excitation electrodes 28 are shownin FIG. 1 as having uniform shape and spacing, the stripes mayalternatively have varying sizes and/or pitch. Alternatively, any othersuitable electrode arrangements may be used (for example, concentricring-shaped electrodes). In an embodiment of the present invention,excitation electrode 29 (not visible in FIG. 1) is disposed and formedas a uniform layer on substrate 27, functioning as an electrical groundplane. Alternatively, the excitation electrodes on substrate 27 maycomprise stripes, which are typically oriented perpendicularly toelectrodes 28, or may be formed in any other suitable pattern.

After forming excitation electrodes 28 and 29, substrates 26 and 27 arecemented together at a predefined distance, typically a few microns, byusing cements and/or etched spacers as are known in the art. Liquidcrystal material 24 is then inserted and sealed in the gap between thesubstrates. Although for the sake of visual clarity, only a fewexcitation electrodes 28 are shown in FIG. 1, in practice, for goodoptical quality, optical phase modulator 22 will typically comprise atleast 100 stripe electrodes for excitation, and possibly even 400 ormore.

Control circuitry 23 is coupled to each of excitation electrodes 28 and29, and is configured to apply control voltage waveforms to theexcitation electrodes for modifying the local optical phase modulationthrough liquid crystal material 24 in response to the local controlvoltage waveform.

The pictured embodiment combining striped excitation electrodes 28 anduniform electrode 29 enables optical device 20 to function as aone-dimensional optical phase modulator. In an embodiment of the presentinvention, optical device 20, as a one-dimensional phase modulator,emulates a cylindrical lens. The focal length of the cylindrical lensand the position of the focal line in a direction orthogonal to the lineare determined by the phase modulation profile, which is induced inliquid crystal material 24 by the control voltage waveforms that controlcircuitry 23 applies to excitation electrodes 28. Alternatively, othercontrol voltage waveforms may be applied to emulate lenses yieldingother one-dimensional wavefronts, including free-form one-dimensionallenses.

In some embodiments of the present invention, a two-dimensional opticalphase modulator (not shown) is assembled from two identical or similarone-dimensional phase modulators 22 by stacking them in series, with thedirections of their striped excitation electrodes 28 orthogonal to eachother. Control circuitry 23 is in this embodiment coupled to bothoptical phase modulators 22. The control voltage waveforms applied bycontrol circuitry 23 across excitation electrodes 28 and 29 of the twooptical phase modulators 22 may be chosen so as to yield a phasemodulation profile that is circularly symmetrical, thus emulating aspherical lens. The focal length, as well as the position of the focalspot in the focal plane, may be adjusted by the control voltagewaveforms applied by control circuitry 23 across each of a set ofexcitation electrodes 28 and 29 of the two optical phase modulators 22.Alternatively, different, symmetrical or non-symmetrical, patterns ofcontrol voltage waveforms may be applied so that the combination of twoorthogonal optical phase modulators 22 emulates, for example, anastigmatic lens, an aspheric lens, a toric lens, a lenslet array, or afree-form lens.

In an alternative embodiment of the present invention, as mentionedabove, excitation electrodes 29 are formed as parallel stripes, similarto excitation electrodes 28, but running in a direction orthogonal toexcitation electrodes 28. This embodiment enables a single optical phasemodulator 22 to function as a two-dimensional phase modulator. Whensuitable control voltage waveforms are applied by controller 23 acrossexcitation electrodes 28 and 29, optical phase modulator 22 may emulate,for example, a spherical lens, an astigmatic lens, an aspheric lens, atoric lens, a lenslet array, or a free-form lens. By applying a constantcontrol voltage waveform by controller 23 to all of excitationelectrodes 28 or to all of excitation electrodes 29, optical phasemodulator 22 reverts to functioning as a one-dimensional phasemodulator, whose functions were described above.

Similarly to excitation electrodes 28, the stripes of excitationelectrodes 29 may alternatively have varying sizes and/or pitch, or anyother suitable electrode arrangements may be used.

Control circuitry 23 typically comprises amplifiers and/or switches, asare known in the art, which control either the amplitude or the dutycycle, or both, of the voltage that is applied to each of electrodes 28and 29. The pattern of amplitudes and/or duty cycles applied to theexcitation electrodes determines the phase modulation profile of liquidcrystal material 24. The circuit components in control circuitry 23 aretypically fabricated as a silicon chip. Control circuitry 23 may belocated separately from optical phase modulator 22, and connected toeach of excitation electrodes 28 and 29 by suitable bonding wires orother connections, as is shown in FIG. 1. Alternatively, controlcircuitry 23 may be cemented onto one of substrates 26 or 27, andconnected to each of excitation electrodes 28 and 29 by suitable bondingwires or other connections (not shown). Control circuitry 23 can belocated at the side of the array of excitation electrodes 28 or 29, andthere is no need for any parts of the control circuitry to be locatedover the active area of layer 24.

Control circuitry 23 is configured to modify the control voltagewaveforms applied to each of excitation electrodes 28 and 29concurrently and independently. This concurrent driving may apply to allof the electrodes as a group (in which case the control voltagewaveforms of all the electrodes are updated together) or to sub-groupsof the electrodes. For example, control circuitry 23 may update thecontrol voltage waveforms applied to all the odd excitation electrodesin the array alternately with all the even excitation electrodes. Thissort of approach scales readily to large electrode counts, and can thusbe used to create electrically-tunable optical systems with high pixelcounts and fine resolution.

FIG. 2 is a schematic perspective view of the structure of heliconicalliquid crystal material 24, in accordance with an embodiment of thepresent invention. Heliconical (also known as “oblique helicoidal”)liquid crystal material 24 has recently been reported by Xiang et al. in“Electrically tunable selective reflection of light from ultraviolet tovisible and infrared by heliconical cholesterics”, Advanced Materials27, pp. 3014-3018 (2015).

A chain 34 of molecules 36 of heliconical liquid crystal material 24 isshown schematically between substrates 26 and 27. A director {circumflexover (n)}, which is a unit vector 37 along the local orientation of themolecules of liquid crystal material 24, rotates around a helicoidalaxis 38 as it follows chain 34 of molecules 36, defining a tilt angle θwith the axis, also referred to as the molecule director angle. Forheliconical liquid crystal material 24, tilt angle θ is less than 90°(as opposed to CLC, where θ=90°). A pitch P, denoted by a double-arrow39, of heliconical liquid crystal material 24 can be very short, forexample less than 250 nm. A break 40 in chain 34 indicates that inreality there are many more periods of heliconical liquid crystalmaterial 24 (from tens to hundreds of periods between substrates 26 and27 than the 1-2 periods that are drawn in FIG. 2. Due to the shorthelical pitch P, heliconical liquid crystal material 24 ispolarization-independent for light that enters device 20 alonghelicoidal axis 38.

Another advantage of using a pitch P that is less than 250 nm relates toBragg-reflections from the periodic structure of heliconical liquidcrystal material 24. Bragg-reflections take place for light incident onthe liquid crystal along helicoidal axis 38 in a spectral band centeredat a so-called Bragg-wavelength λ_(Bragg)=n_(ave)×P, wherein n_(ave) isthe average refractive index of the heliconical liquid crystal material.Using a typical value of n_(ave)=1.65 and a pitch P=200 nm, theBragg-wavelength is λ_(Bragg)=330 nm. The reflected light is well in theultra-violet (UV) region of the spectrum, ensuring that incident lightin the visible spectrum will pass through optical phase modulator 22without significant losses from Bragg-reflections.

Heliconical liquid crystal material 24, as suggested by Xiang et al.,for example, comprises a mixture of two dimeric liquid crystals:(1′,7′-bis(4-cyanobiphenyl-4′-yl)heptane (CB7CB) and1-(4-cyanobiphenyl-4′-yl)-6-(4cyanobiphenyl-4′-yloxy)hexane (CB6OCB)),and a standard liquid crystal, pentylcyanobiphenyle (5CB). The mixtureis doped with a left-handed chiral additive S811, which determines thepitch. All of the above components of heliconical liquid crystalmaterial 24 are manufactured by Merck & Co., Inc., Kenilworth, N.J.,USA. A possible mixture composition CB7CB:CB6OCB:5CB:S811 (in weightunits) is 30:20:45:5 (cholesteric phase in the range 20.0° C.-66.5° C.).Alternatively, other sorts of achiral dimer molecules of the type CBnCB(of which CB7CB is one example) may be used to form chiral heliconicalliquid crystal materials, as reported by Chen et al., in “Chiralheliconical ground state of nanoscale pitch in a nematic liquid crystalof achiral molecular dimers,” Proceedings of the National Academy ofSciences of the U.S.A. 110(40), pages 15931-15936 (2013).

Another example of a liquid crystal that can be used in material 24 isUD68, wherein the liquid crystal is achiral but the twist-bend nematicphase is chiral, as reported by Chen et al., in “Twist-bend heliconicalchiral nematic liquid crystal phase of an achiral rigid bent-coremesogen,” Physical Review E 89, page 22506 (2014).

Alternatively, other mixtures of liquid crystals with or without chiraladditives (left- or right-handed) may be used to create heliconicalliquid crystal materials with pitch in the desired range, and suchmixtures are considered to be within the scope of the present invention.

Applying an electrical field along helicoidal axis 38 has the effect ofdecreasing both tilt angle θ and pitch P, thus modifying the effectiverefractive index. However, as opposed to CLC, the helical structure ofheliconical liquid crystal material 24 is preserved under the appliedelectrical field, and the material exhibits a polarization-independentoptical path length, which varies with the varying electrical field.This feature enables a continuous and polarization-independent phasemodulation in optical device 20.

An additional advantage of heliconical liquid crystal material 24 isthat, by decreasing pitch P by an applied electrical field, thewavelength λ_(Bragg) for Bragg-reflections, mentioned above, may bemoved to ultraviolet (UV) wavelengths in case the zero-field pitchresults in Bragg-reflection in the visible spectrum.

Polarization-Independence Using Multiple Lenses

As noted earlier, a polarization-independent lens may be assembled bystacking two polarization-dependent lenses. The twopolarization-dependent lenses are designed to affect polarized light inpolarizations P1 and P2 respectively, wherein P1 and P2 are orthogonal.The first lens operates as a lens for P1 polarized light, but does notaffect light in the orthogonal P2 polarization. Similarly, the secondlens operates as a lens for P2 polarized light, but does not affectlight in the orthogonal P1 polarization. Therefore, the combination ofthe two lenses operates as one polarization-independent lens.

A lens positioned opposite an eye causes a magnification M of the imageon the retina, compared to an image created on the retina without acorrective lens. For positive lenses the image is magnified (M>1), whilefor negative powers the image size is reduced (M<1). The magnificationdepends not only on the focal length of the lens, but also on thedistance between the pupil and the lens. A larger distance between thepupil and the lens, as well as a shorter focal length of the lens, willresult in a larger effect on the magnification.

When the polarization-independent lens described above is placedopposite an eye, the two lens elements composing the device arepositioned at different distances from the pupil. Therefore, the imageon the retina will be constructed of P1 polarized light focused by thefirst lens element with magnification M1, and P2 polarized light focusedby the second lens element with magnification M2. If M1 and M2 are notequal, this can result in a doubling of the image of the retina, thuslowering the perceived sharpness and overall quality of the image.

In an embodiment of the present invention, the focal lengths of the twolenses are controlled so that their difference compensates for thedistance separating the lenses so as to equalize the magnifications M1and M2 for the retinal images at the two polarizations P1 and P2,respectively. Since magnification cannot be controlled independentlyfrom focus, equalizing the magnifications M1 and M2 implies that one orboth of the retinal images at the two polarizations P1 and P2 are not ina sharp focus. Equalizing the magnifications rather than focusing bothpolarizations is preferable, as a slight loss of image resolution (withequal magnifications) is tolerated better by the human visual perceptionthan a sharp double image.

FIG. 3 is a schematic perspective view of a polarization-independentoptical device 120, in accordance with another embodiment of the presentinvention. Optical device 120 comprises two polarization-dependentlenses 121 a and 121 b, each comprising an optical phase modulator 122and control circuitry 123. Although control circuitry 123 is shown as aunitary component, it could as well comprise a separate component foreach of the two lenses 121 a and 121 b. Optical phase modulator 122comprises an electro-optical layer 124, which typically comprisesnematic or other birefringent liquid crystal material, sandwichedbetween an upper substrate 126 and a lower substrate 127, wherein thesubstrates comprise a transparent material, for example, glass. Theinner surfaces of upper substrate 126 and lower substrate 127 can becoated with an alignment layer, such as polyimide, as referred to above.

Optical phase modulator 122 of lens 121 b is similar to the opticalphase modulator of lens 121 a. Electro-optical layer 124 is typicallycontained by suitable encapsulation, as is known in the art.

Light impinges on optical device 120 as an incident light 130, and exitsthe phase modulators as a transmitted light 132. The optical phase oftransmitted light 132 is locally modified, with respect to the opticalphase of incident light 130, by the local optical path throughelectro-optical layers 124. The local optical path is modified inresponse to applied control voltage waveforms, as will be detailedbelow. As electro-optical layers 124 are typically birefringent, themodification of the optical phase in each of lenses 121 a and 121 baffects only one polarization of transmitted light 132. Therefore,lenses 121 a and 121 b are arranged so that the respectiveelectro-optical layers 124 operate on orthogonal polarizations of light.

Excitation electrodes 128 and 129 are disposed over substrates 126 and127, respectively. Control circuitry 123 is coupled to excitationelectrodes 128 and 129, and is configured to apply control voltagewaveforms to the excitation electrodes, modifying the local optical paththrough electro-optical layers 124 according to the local controlvoltage waveform. The features of electrodes 28 and 29 and controlcircuitry 23 that were described above are likewise applicable, mutatismutandis, to electrodes 128 and 129 and control circuitry 123.

FIGS. 4A-B are schematic side views of the use of twopolarization-dependent lenses 140 and 142 as a polarization-independentophthalmic lens 144, in accordance with an embodiment of the presentinvention. Lenses 140 and 142 are constructed and operate, for example,in accordance with the principles of device 120 described above.

FIG. 4A shows an eye 146 viewing an object point 148 with the aid ofpolarization-independent ophthalmic lens 144. In the present example,object point 148 is located 33 cm to the left from a pupil 150 of eye146, and 3 cm above an optical axis 152 of the eye, although otherdistances and heights of the object point may be handled in similarfashion. Polarization-independent ophthalmic lens 144, comprisingpolarization-dependent lenses 140 and 142 (shown in FIG. 4B), is locatedin front of eye 146. Optical rays 154 travel from object point 148through polarization-independent ophthalmic lens 144 into eye 146through its pupil 150, and image the object point onto a retina 151 ofthe eye. Polarization-independent ophthalmic lens 144 is, through itscomponent lenses 140 and 142, coupled to control circuitry 156.

FIG. 4B is a schematic raytrace of optical rays 154 from object point148 (outside FIG. 4B, but shown in FIG. 4A) traversingpolarization-dependent lenses 140 and 142 and continuing to eye 146.Polarization-dependent lenses 140 and 142, with orthogonal polarizationaxes, together make up polarization-independent ophthalmic lens 144,shown by a double dotted line. Lens 140 refracts rays of polarization P1and lens 142 refracts rays of polarization P2. For clarity,polarization-dependent lenses 140 and 142 are drawn schematically asthin lenses, and eye 146 is represented schematically as a pupil plane158 (representing pupil 150) and retina 151.

In the present example, polarization-dependent lenses 140 and 142 arelocated at respective distances of 2.2 cm and 2.0 cm to the left ofpupil plane 158. The optical centers of both polarization-dependentlenses 140 and 142 are shifted to 1 cm below optical axis 152 of eye146, defining an optical axis 160 for polarization-independentophthalmic lens 144. Rays 154, shown as double solid lines, arrive fromobject point 148 impinging first on lens 140. Lens 140 refracts rays ofpolarization P1, which then pass through lens 142 without refracting.Rays of polarization P2 are not refracted by lens 140, but are refractedby lens 142. Rays 154 are, through refraction, separated by lenses 140and 142 into rays 162 of polarization P1 (drawn as dotted lines) andrays 164 of polarization P2 (drawn as solid lines). Rays 162 and 164 arerefracted at pupil plane 158, and image object point 148 onto retina151.

Control circuitry 156 has adjusted the optical powers of lenses 140 and142 to 3.25 D (D=diopters) and 3.3 D, respectively. With these opticalpowers object point 148 is imaged with the same magnification for thetwo polarizations P1 and P2. Imaging of object point 148 is shown ingreater detail by expanding an area 166 to an area 168. Rays 162 ofpolarization P1 arrive at a point 172 on retina 151 in a sharp focus.Rays 164 of polarization P2 arrive at point 172 with a slight defocus.Both rays 162 and 164 are centered at or around point 172, which isequivalent to equal magnification of imaging for both polarizations P1and P2. Due to its small value, the blur of rays 164 (polarization P2)has no significant effect on the visual perception of object point 148.Alternatively, the blur diameter could be further reduced by adjustingthe optical powers of lenses 140 and 142 so that the images at bothpolarizations P1 and P2, still having equal magnification, are defocusedby the same amount. However, were the optical powers of both lenses 140and 142 adjusted by control circuitry 156 for a sharp focus on retina151 (in the disclosed embodiment to 3.25 D for lens 140 and to 3.22 Dfor lens 142), the lateral distance between the two sharp images ofobject point 148 on the retina would be 15 μm, which would be perceivedas an objectionable double image.

Polarization-independent ophthalmic lens 144, as well aspolarization-dependent lenses 140 and 142, are described in thedisclosed embodiment as positive lenses. Alternatively, lenses 140 and142 may similarly be configured as negative lenses, and the principlesexplained above may similarly be applied in adjusting the respectiveoptical powers.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

1. An optical device, comprising: an electro-optical layer, comprising aliquid crystal material with a heliconical structure having a moleculedirector angle that is less than 90° and a pitch that is less than 250nm and is modifiable by an electric field; an array of excitationelectrodes extending over the electro-optical layer; and controlcircuitry, which is coupled to apply control voltage waveforms to theexcitation electrodes and is configured to modify the control voltagewaveforms so as to locally modify the molecule director angle of theheliconical structure and thus to generate a specified phase modulationprofile in the electro-optical layer.
 2. The optical device according toclaim 1, wherein the liquid crystal material comprises a combination ofone or more liquid crystals and a chiral additive.
 3. The optical deviceaccording to claim 1, wherein the liquid crystal material comprises achiral liquid crystal.
 4. The optical device according to claim 1,wherein the phase modulation profile is independent of a polarization ofthe incident light.
 5. The optical device according to claim 1, whereinthe control circuitry is configured to apply the control voltagewaveforms to the excitation electrodes so that the device functions as alens, having focal properties determined by the phase modulationprofile.
 6. The optical device according to claim 5, wherein the lens isconfigured as an ophthalmic lens with an electrically controllable focallength. 7-10. (canceled)
 11. A method for producing an optical device,the method comprising: providing an electro-optical layer, comprising aliquid crystal material with a heliconical structure having a moleculedirector angle that is less than 90° and a pitch that is less than 250nm and is modifiable by an electric field; positioning an array ofexcitation electrodes to extend over the electro-optical layer; andcoupling control circuitry to apply control voltage waveforms to theexcitation electrodes and to modify the control voltage waveforms so asto locally modify the molecule director angle of the heliconicalstructure and thus to generate a specified phase modulation profile inthe electro-optical layer.
 12. The method according to claim 11, whereinthe liquid crystal material comprises a combination of one or moreliquid crystals and a chiral additive.
 13. The method according to claim11, wherein the liquid crystal material comprises a chiral liquidcrystal.
 14. The method according to claim 11, wherein the phasemodulation profile is independent of a polarization of the incidentlight.
 15. The method according to claim 11, wherein coupling thecontrol circuitry comprises applying the control voltage waveforms tothe excitation electrodes so that the device functions as a lens, havingfocal properties determined by the phase modulation profile.
 16. Themethod according to claim 15, and comprising configuring the lens as anophthalmic lens with an electrically controllable focal length. 17-20.(canceled)
 21. The optical device according to claim 1, whereinapplication of the control voltage waveforms locally modifies the pitchof the heliconical structure.
 22. The optical device according to claim5, wherein the focal properties of the lens vary continuously inresponse to a variation of an electric field applied by the controlvoltage waveforms.
 23. The method according to claim 11, whereinapplication of the control voltage waveforms locally modifies the pitchof the heliconical structure.
 24. The method according to claim 15,wherein the focal properties of the lens vary continuously in responseto a variation of an electric field applied by the control voltagewaveforms.